JP7367445B2 - Electrode for reduction reaction, method for manufacturing electrode for reduction reaction, and reaction device using electrode for reduction reaction - Google Patents

Description

The present invention relates to a reduction reaction electrode, a method for manufacturing the reduction reaction electrode, and a reaction device using the reduction reaction electrode.

Using solar energy, we can convert water (H ₂ O) into hydrogen (H ₂ ), water (H ₂ O) and carbon dioxide (CO ₂ ) into carbon monoxide (CO), formic acid (HCOOH), and methanol (CH ₃ OH). ) and the like are disclosed for reaction devices used in artificial photosynthesis. In order to put this into practical use, it is desired to realize a reaction electrode that can cause a reaction with high efficiency.

In the reaction device, a reduction reaction electrode and an oxidation reaction electrode are used in combination. For example, Patent Document 1 describes a substrate containing a metal or a semiconductor, a carbon layer that covers at least a part of the surface of the substrate and is made of a carbon material and has a structure of 1 μm or less in size, and a carbon layer that covers at least a part of the surface of the substrate, and A reduction reaction electrode is disclosed, which includes a covering complex catalyst layer.

As a catalyst for a reduction reaction electrode, Ru is polymerized using an Ru complex polymer solution containing a Ru complex monomer, a polymerization initiator (e.g., pyrrole), a polymerization catalyst (e.g., iron chloride), and a solvent. Complex polymers are known. Materials used for reduction reaction electrodes of reaction devices used in artificial photosynthesis, etc. and electrode manufacturing methods are described to some extent in Patent Document 1 and Non-Patent Document 1, etc., but in particular, the handling of Ru complex polymer solutions is disclosed. There is no mention of such things, and their condition and properties are unknown.

Reduction reaction electrodes are required to have a large area and high output, and in the production of reduction reaction electrodes using this Ru complex polymer solution, reduction reaction electrodes that can have a large area and high output are required. is required to obtain.

Japanese Patent Application Publication No. 2017-125242

Takeo Arai, Shunsuke Sato and Takeshi Morikawa, A monolithic device for CO2 photoreduction to generate liquid organic substances in a single-compartment reactor, Energy Environ. Sci., 2015, 8, 1998

An object of the present invention is to provide a reduction reaction electrode that uses a Ru complex polymer and can have a large area and high output, a method for manufacturing the reduction reaction electrode, and a reaction device using the reduction reaction electrode. There is a particular thing.

The present invention has a catalyst layer comprising: a base material; a composite base material of carbon fibers and carbon; and a Ru complex polymer; graphite ( 002) An electrode for reduction reaction having an X-ray peak intensity of 100 CPS or more.

The present invention has a catalyst layer comprising: a base material; a composite base material of carbon fibers and carbon; and a Ru complex polymer; graphite ( 002) An electrode for reduction reaction, in which the X-ray peak intensity is 0.02 or more relative to the graphite (002) X-ray peak intensity in X-ray diffraction analysis of the composite base material by the θ-2θ method .

The present invention provides a method for producing the electrode for reduction reaction, which includes a preparation step of preparing a Ru complex polymer solution containing a Ru complex monomer, a polymerization catalyst, and a solvent, and a step of preparing the Ru complex polymer solution after preparation. This is a method for producing an electrode for reduction reaction, which includes a storage step of storing at 5° C. for 5 to 15 hours, and a coating step of applying the Ru complex polymer solution after storage to the composite substrate and drying it.

The present invention is a reaction device configured by combining the reduction reaction electrode and the oxidation reaction electrode.

ADVANTAGE OF THE INVENTION According to the present invention, it is possible to provide a reduction reaction electrode that uses a Ru complex polymer and can have a large area and high output, a method for manufacturing the reduction reaction electrode, and a reaction device using the reduction reaction electrode. can.

1 is a schematic configuration diagram showing an example of the configuration of a reduction reaction electrode according to an embodiment of the present invention. 1 is a schematic configuration diagram showing an example of the configuration of a reaction device (artificial photosynthesis apparatus) using a reduction reaction electrode according to an embodiment of the present invention. 2 is a graph showing the relationship between the current value (n=2) after 3 hours of IT measurement operation and the storage time after preparing the Ru complex polymer solution in Example 1 and Comparative Example 1. 2 is a graph showing a comparison of the IT characteristics of the reduction reaction electrode of the Ru complex polymer solution stored in a refrigerator, at room temperature for 10 minutes, and for 5 hours in Example 1 and Comparative Example 1. 3 is a graph showing a comparison of the IT characteristics of the Ru complex polymer solution stored at room temperature for 5 hours with and without light shielding in Reference Example 1. 1 is a photograph showing a SEM image of a typical reduction reaction electrode after experiments in Example 1 and Comparative Example 1. 1 is a graph showing an XRD pattern of a typical reduction reaction electrode after experiments in Example 1 and Comparative Example 1. It is a graph showing the relationship between the X-ray peak intensity of the (002) plane of graphite and the storage time of the Ru complex polymer solution of the electrode. 9 is a graph showing normalized values based on the peak intensity of 5000 CPS of graphite (002) shown in FIG. 8 as a base (1.0). 3 is a graph showing the relationship between the current value after 3 hours of IT measurement and the X-ray peak intensity of the (002) plane of graphite. It is a graph showing the relationship between the current value after 3 hours of IT measurement operation and the value normalized using the peak intensity of graphite (002) of 5000 CPS as a base (1.0). It is a graph showing the relationship between the current value after 3 hours of IT measurement operation and the value normalized using the peak intensity of graphite (002) of 5000 CPS as a base (1.0).

Embodiments of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

[Electrode for reduction reaction]
FIG. 1 is a schematic configuration diagram showing an example of the configuration of a reduction reaction electrode according to an embodiment of the present invention. The reduction reaction electrode 10 in FIG. 1 has a base material 12; a catalyst layer 16 containing a composite base material of carbon fiber and carbon, and a Ru complex polymer. The catalyst layer 16 may be bonded to the base material 12 by an adhesive layer 14. When a voltage is applied to the reduction reaction electrode 10, the electric charge generated in the base material 12 is transferred to the catalyst layer 16, and a reduction catalytic reaction occurs in the catalyst layer 16, for example, carbon dioxide (CO ₂ ) is converted to formic acid (HCOOH). Used in reduction reactions.

The base material 12 is a member that structurally supports the electrode. Although the material of the base material 12 is not particularly limited, examples thereof include a glass substrate and the like. The base material 12 may include, for example, a metal or a semiconductor. The metal used as the base material is not particularly limited, but examples include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), and tin. (Sn), palladium (Pd), lead (Pb), etc. The semiconductor used as the base material is not particularly limited, but includes, for example, titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), and zinc oxide (ZnO 2 ). ), tantalum oxide (Ta ₂ O ₅ ), and the like. When the base material contains a metal or a semiconductor, an insulating layer may be formed between the catalyst layer and the base material. The insulating layer is not particularly limited, but includes semiconductor oxides, nitrides, resins, and the like. A structure may be adopted in which the metal substrate and the catalyst layer are directly electrically connected.

The base material 12 may include carbon fiber. The base material 12 is, for example, a composite base material of carbon fiber and carbon that has been heat-treated at a high temperature, and examples thereof include carbon paper, carbon cloth, and the like. Carbon paper is, for example, a sheet made by impregnating organic fibers such as polyacrylonitrile (PAN) fibers in a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing and bonding them at about 2000°C. . Carbon paper may contain about 25% by mass of Teflon material. Carbon cloth is made by spinning carbon fibers obtained by firing and carbonizing organic fibers. Carbon paper and carbon cloth are porous bodies and have countless pores of several tens of μm (about 10 μm to 100 μm). The thickness of carbon paper and carbon cloth is, for example, in the range of 0.1 mm to 0.4 mm/sheet. This composite base material may be multilayered (for example, 5 to 10 layers) and have a thickness approximately 10 times greater, with a thickness in the range of 1 mm to 4 mm. The multilayered composite substrate may be formed by, for example, using a carbon-based adhesive containing a conductive carbon material, such as graphite or a graphene-containing polymer (e.g., an acrylic polymer binder) as an adhesive, to form a plurality of carbon papers, It can be formed by bonding and laminating carbon cloth or the like.

By using a composite base material of carbon fiber and carbon as the base material 12 for the reduction reaction electrode, it becomes possible to have multiple layers, a large area, and a high output. Since the base material has excellent compression properties and is lightweight, it is easy to stack the electrodes in the direction of the surface to form multiple layers (for example, 10 or more layers). Therefore, higher output (higher current) of the reaction device can be achieved. Since the composite base material of carbon fiber and carbon is lightweight, the entire electrode can be made lighter and can easily be made larger in area. In addition, the composite base material of carbon fiber and carbon can ensure high conductivity, and since it is porous, the flow of reaction substrates and reaction products is good, and side reactions such as hydrogen generation are prevented. It is thought that suppression and improvement of the reduction reaction efficiency of carbon compounds can be achieved. Furthermore, it has excellent corrosion resistance even in an electrolytic solution having a pH of 6.5 to 7, for example. Furthermore, the surface smoothness is also excellent because the smoothness in the surface direction can be easily improved by, for example, filling the gaps in the composite base material with a carbon material such as carbon.

The catalyst layer 16 can be bonded onto the base material 12 via the adhesive layer 14. At this time, it is preferable to use a conductive carbon material, such as a carbon-based adhesive containing graphite or a polymer containing graphene (eg, an acrylic polymer binder), as the adhesive. It is preferable that the adhesive layer 14 contains, for example, graphite or graphene having a diameter of at least 1 μm or more and 100 μm or less.

The catalyst layer 16 contains a Ru complex polymer as a reduction catalyst for a reduction reaction such as a carbon dioxide reduction reaction. The catalyst layer 16 may be one in which a reduction catalyst is supported on a composite base material of carbon fibers and carbon, or one in which a reduction catalyst layer containing a reduction catalyst is formed on a composite base material of carbon fibers and carbon. good. When a composite base material of carbon fibers and carbon is used as the base material 12, by using a composite base material of carbon fibers and carbon for the catalyst layer 16, electricity is It is thought that the chemical properties will be stable and high current will be possible.

As the ruthenium complex (Ru complex monomer) for obtaining the Ru complex polymer, for example, [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)( MeCN)Cl ₂ ], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO) ₂ Cl ₂ ], [Ru{4,4'-di (1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO) ₂ ] _n , [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'- bipyridine}(CO)( _CH3CN ) _Cl2 ], and the like. For example, the Ru complex polymer can be polymerized by initiating polymerization with a Ru complex monomer (for example, [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO) ₂ Cl ₂ ]). It is a Ru complex polymer that has been polymerized using an agent (for example, pyrrole) and a polymerization catalyst (for example, iron chloride).

The Ru complex polymer can be supported by, for example, dissolving a Ru complex monomer, a polymerization initiator (e.g. pyrrole), and a polymerization catalyst (e.g. iron chloride) in a solvent such as acetonitrile (MeCN) (Ru complex polymer solution) with carbon fibers. This can be done by coating on a composite base material with carbon and drying it.

As a polymerization initiator (radical initiator), a heterocyclic aromatic compound having a five- to nine-membered ring structure containing at least one of N, S, and B, such as pyrrole, pyridine, thiophene, volepine, and azonine; Examples thereof include compounds, azo compounds, organic peroxides, etc., and pyrrole is preferable because the reaction is easily initiated by oxidation.

Examples of polymerization catalysts (chemical oxidation polymerization catalysts) include iron salts such as iron chloride (FeCl ₃ ), FeCl ₃ .O ₂ , or FeCl ₃ .O ₂ -ClO ₄ , copper salts such as copper chloride, and aluminum such as aluminum chloride. Iron chloride (FeCl ₃ ), FeCl ₃ .O ₂ , or FeCl ₃ .O ₂ -ClO ₄ is preferable from the viewpoint of high catalytic activity.

Examples of the solvent include acetonitrile, diethyl ether, N,N-dimethylformamide (DMF), and tetrahydrofuran (THF), with acetonitrile being preferred since it can highly disperse the Ru complex polymer.

The catalyst layer 16 may include a carbon material such as carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs). By including a carbon material such as multi-wall carbon nanotubes (MWCNTs), a three-dimensional network structure of nanocarbons can be formed. It is preferable that the multi-wall carbon nanotubes include those having a diameter of at least 1 nm or more and 100 nm or less. Carbon materials such as multi-walled carbon nanotubes (MWCNTs) are prepared by preparing an ink in which the carbon material is highly dispersed in a solvent such as ethanol, applying it by a method such as dip coating or impregnation coating, and drying it. It can be included in the catalyst layer 16.

In the process of manufacturing a reduction reaction electrode for a large cell for artificial photosynthesis using a Ru complex polymer solution containing a Ru complex monomer, a polymerization initiator (e.g., pyrrole), a polymerization catalyst (e.g., iron chloride), and a solvent, by scaling up , various concerns were observed. For example, large amounts of Ru complex monomer are required for scale-up. Therefore, if the Ru complex monomer is stored in the atmosphere for a long time, the Ru complex deteriorates and the reduction characteristics of the reduction reaction electrode deteriorate.However, the Ru complex monomer is stored in an inert gas atmosphere. If properly managed, the effect on cell characteristics could be suppressed. However, in the process of supporting the Ru complex polymer on a composite base material of carbon fiber and carbon using a Ru complex polymer solution, the process of applying the Ru complex polymer solution and drying process is repeated many times on the composite base material for large cells. If repeated, the entire process takes time, and during that time, polymerization of pyrrole progresses after the preparation of the Ru complex polymer solution, and the color may change from orange to black. Therefore, there was concern that the degree of polymerization of the Ru complex polymer solution would be different, and that the morphology of the Ru complex polymer after being applied to the composite substrate and drying would be different depending on the time the Ru complex polymer solution was allowed to stand after preparation.

Research has been conducted on the polymerization of pyrrole for a long time, and it has been found that electrolytic polymerization (which can be directly attached to an electrode in the form of a film) occurs through electrochemical oxidation using an inert electrode. Chemical polymerization of pyrrole also occurs in the process of producing the present reduction reaction electrode by adding a polymerization catalyst such as iron chloride. Therefore, if the Ru complex polymer solution is left to stand after preparation, the polymerization of pyrrole will proceed, and the degree of polymerization of the Ru complex polymer will vary depending on the standing time, causing a problem that the reduction characteristics of the reduction reaction electrode will change.

The present inventors have developed a method for manufacturing a reduction reaction electrode by treating the storage time and storage temperature after preparation of the Ru complex polymer solution as predetermined ranges, that is, the optimal storage time and storage temperature ranges. The inventors have discovered that it is possible to produce a stable reduction reaction electrode with good electrochemical properties. Then, the graphite (002) We found that there is a correlation between In this optimal storage time and storage temperature range, even a Ru complex polymer solution whose capacity has been increased in scale-up can be stably handled, which is an important condition for producing an electrode for reduction reaction.

In the reduction reaction electrode 10 according to the present embodiment, the graphite (002) X-ray peak intensity in X-ray diffraction analysis of the catalyst layer 16 is 100 CPS or more, preferably 150 CPS or more. If the graphite (002) X-ray peak intensity in the X-ray diffraction analysis of the catalyst layer 16 is less than 100 CPS, the obtained current will decrease. The upper limit of this graphite (002) X-ray peak intensity is not particularly limited, but is, for example, 850 CPS or less.

In addition, in the reduction reaction electrode 10, the graphite (002) X-ray peak intensity in the X-ray diffraction analysis of the catalyst layer 16 is greater than the graphite (002) X-ray peak intensity in the X-ray diffraction analysis of the composite base material. , the value normalized using the graphite (002) X-ray peak intensity as a base (1.0) is 0.02 or more, preferably 0.03 or more. If this normalized value is less than 0.02, the obtained current will decrease. The upper limit of this normalized value is not particularly limited, but is, for example, 0.17 or less.

In the process of preparing the Ru complex polymer solution, chemical polymerization of pyrrole with a polymerization catalyst such as iron chloride occurs, and by maintaining the optimum polymerization time and temperature, composite substrates (e.g. CP/MWCNTs sheets) can be produced. It is thought that an electrode for a reduction reaction that can support and dry the Ru complex polymer in an optimal state and ensure a high current can be obtained.

The method for manufacturing a reduction reaction electrode according to the present embodiment includes a preparation step of preparing a Ru complex polymer solution containing a Ru complex monomer, a polymerization catalyst, and a solvent, and a step of preparing the Ru complex polymer solution after the preparation at -15 to 5°C. It includes a storage step of storing for 5 to 15 hours, and a coating step of applying the Ru complex polymer solution after storage to the composite base material and drying it. After the coating step, the method may further include a washing step of immersing the composite substrate in ultrapure water and then drying it at 60 to 120°C. The cleaning step can reduce the amount of residual polymerization catalyst and the like.

For example, a composite base material/carbon material sheet (for example, CP/MWCNTs sheet) can be obtained by impregnating and coating a composite base material of carbon fibers and carbon with a carbon material dispersion in which a carbon material such as MWCNTs is dispersed in a solvent, and drying it. get. On the other hand, a Ru complex polymer solution containing a Ru complex monomer, a polymerization catalyst (for example, iron chloride), a polymerization initiator (for example, pyrrole), and a solvent is prepared (preparation step). The prepared Ru complex polymer solution is stored at -15 to 5°C for 5 to 15 hours (storage step). The Ru complex polymer solution after storage is applied to a composite base material/carbon material sheet (eg, CP/MWCNTs sheet) (coating step). The Ru complex polymer solution is dried on the composite base material/carbon material sheet by vacuum drying or the like to obtain a composite base material/carbon material/catalyst sheet (for example, CP/MWCNTs/RuCP sheet). This composite base material/carbon material/catalyst sheet (for example, CP/MWCNTs/RuCP sheet) is adhesively bonded to the base material by an adhesive layer containing, for example, a polymer and a carbon material such as graphite, and the base material 12; It is possible to obtain a reduction reaction electrode 10 having a composite base material of carbon fibers and carbon, and a catalyst layer 16 (for example, CP/MWCNTs/RuCP) containing a Ru complex polymer.

In the storage step, the prepared Ru complex polymer solution is stored at -15 to 5°C for 5 to 15 hours, preferably at 0 to 5°C for 12 to 15 hours. If the storage time and storage temperature are outside this range, the reduction current value of the reduction reaction electrode will decrease.

[Reaction device]
A reaction device according to an embodiment of the present invention is a reaction device configured by combining the above-mentioned reduction reaction electrode and oxidation reaction electrode. The reaction device can be, for example, a carbon dioxide reduction device, and can also be an artificial photosynthesis device that combines the carbon dioxide reduction device and a solar cell.

In the carbon dioxide reduction device, for example, the reduction reaction electrode that advances the reduction reaction of carbon dioxide and the oxidation reaction electrode that is electrically connected to the reduction reaction electrode and causes the oxidation reaction are spaced apart and opposed to each other. This is an electrochemical reaction device having a flow path through which an electrolytic solution containing a reaction substrate (carbon dioxide) flows between an electrode for a reduction reaction and an electrode for an oxidation reaction. The carbon dioxide reduction device includes, for example, the above-mentioned reduction reaction electrode that advances the reduction reaction of carbon dioxide, and an oxidation reaction electrode that is electrically connected to the reduction reaction electrode and causes the oxidation reaction, and It may be an apparatus in which the reaction electrode and the oxidation reaction electrode are immersed in an electrolytic solution containing a reaction substrate (carbon dioxide).

The artificial photosynthesis device is a device that includes the above carbon dioxide reduction device and a solar cell that generates electric power to be supplied to the reduction reaction electrode and the oxidation reaction electrode of the carbon dioxide reduction device.

FIG. 2 is a schematic configuration diagram showing an example of the configuration of an artificial photosynthesis apparatus as an example of a reaction device using the above electrode for reduction reaction. In the artificial photosynthesis device 1, the reduction reaction electrode 10 for advancing the reduction reaction of carbon dioxide in the storage part 28 and the oxidation reaction electrode 18 for causing the oxidation reaction are arranged at positions facing each other and separated from each other, A carbon dioxide reduction device 3 having a flow path 26 through which an electrolytic solution containing a reaction substrate flows between the reduction reaction electrode 10 and the oxidation reaction electrode 18, and the reduction reaction electrode 10 and the oxidation reaction of the carbon dioxide reduction device 3. This device includes a solar cell 30 that generates power to be supplied to the electrode 18.

In the example of FIG. 2, the reduction reaction electrode 10 and the oxidation reaction electrode 18 each have a structure in which a plurality of electrodes are laminated (stacked). The reduction reaction electrode 10 and the oxidation reaction electrode 18 may each have a single layer structure, or may have a structure in which only the reduction reaction electrode 10 is laminated (stacked), or only the oxidation reaction electrode 18 A structure in which these are laminated (stacked) may also be used. In the case of an artificial photosynthesis device, it is preferable that the reduction reaction electrode 10 and the oxidation reaction electrode 18 each have a structure in which a plurality of electrodes are laminated (stacked).

The artificial photosynthesis device 1 functions by introducing an electrolytic solution containing a reaction substrate such as carbon dioxide into the channel 26 between the reduction reaction electrode 10 and the oxidation reaction electrode 18 from the channel entrance. The electrolyte is discharged from the channel outlet. In the oxidation reaction electrode 18, water (H ₂ O) is oxidized to obtain oxygen (1/2O ₂ ) and electrons are generated. In the reduction reaction electrode 10, by receiving electrons generated by an oxidation reaction, carbon dioxide, for example, is reduced to generate formic acid (HCOOH) or the like.

(Oxidation reaction electrode)
The oxidation reaction electrode 18 is an electrode used to oxidize a substance by an oxidation reaction. The oxidation reaction electrode 18 includes, for example, a base material 20 on which a conductive layer is formed, and an oxidation catalyst layer 22 formed thereon.

The substrate is a member that structurally supports the oxidation reaction electrode 18. Although the material of the substrate is not particularly limited, examples thereof include a glass substrate. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), and tin ( Sn), palladium (Pd), lead (Pb), and the like. The semiconductor used as the substrate is not particularly limited, but includes, for example, titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), and zinc oxide (ZnO). , tantalum oxide (Ta ₂ O ₅ ), and the like. When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but includes semiconductor oxides, nitrides, resins, and the like. A structure may be adopted in which the metal substrate and the conductor layer are directly electrically connected. In order to make the oxidation reaction electrode 18 transparent, the substrate is preferably made of, for example, a glass substrate or plastic. A composite base material of carbon fiber and carbon may be used as the substrate. This has the advantage of ensuring light weight and high conductivity, and eliminating the need for a separate conductive layer.

The conductive layer is provided to effectively collect current at the oxidation reaction electrode 18. The conductive layer is not particularly limited, and examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, in consideration of thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

The oxidation catalyst layer 22 includes a material having an oxidation catalyst function. Examples of materials having an oxidation catalyst function include materials containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of the conductive layer as a nanocolloidal solution (see T. Arai et.al, Energy Environ. Sci 8, 1998 (2015)).

For example, nanocolloids of iridium oxide (IrOx) are synthesized. Next, a 10 wt% sodium hydroxide (NaOH) aqueous solution was added to 50 mL of a 2 mM potassium chloriridate (IV) iridium (K ₂ IrCl ₆ ) aqueous solution to adjust the pH to 13, and the yellow solution was heated at 90°C using a hot stirrer. Heat for 20 minutes. The blue solution thus obtained is cooled with ice water for 1 hour. Then, 3M nitric acid (HNO ₃ ) was added dropwise to the cooled solution (20 mL) to adjust the pH to 1, and the mixture was stirred for 80 minutes to obtain a nanocolloidal aqueous solution of iridium oxide (IrOx). Furthermore, a 1.5 wt% NaOH aqueous solution (1-2 mL) is added dropwise to this solution to adjust the pH to 12. The nanocolloidal aqueous solution of iridium oxide (IrOx) thus obtained is applied onto the conductive layer at a pH of 12, and dried by holding it at 60° C. for 40 minutes in a drying oven. After drying, the precipitated salt can be washed with ultrapure water to form the oxidation reaction electrode 18. Note that the application and drying of the nanocolloid aqueous solution of iridium oxide (IrOx) may be repeated multiple times.

In addition, when using a composite base material of carbon fiber and carbon as a base material in the oxidation reaction electrode 18, for example, a nanocolloidal aqueous solution of iridium oxide (IrOx) is used as an oxidation catalyst to form a multi-wall carbon nanotube. An oxidation catalyst is supported on a composite base material of carbon fibers and carbon such as carbon paper, which may contain carbon materials such as carbon nanotubes (MWCNTs), and the sheet is made of carbon fibers such as carbon paper and carbon. A composite base material bonded with a carbon-based adhesive may also be used.

A current collecting wiring may be provided to enhance the current collecting effect at the oxidation reaction electrode 18. In other words, when the area of the oxidation reaction electrode 18 is increased, the conductivity of the oxidation reaction electrode 18 cannot be ensured by a conductive layer alone to promote a sufficient reaction over the entire surface of the oxidation reaction electrode 18. It is established to increase the For example, the current collection wiring can have a configuration in which linear finger electrodes arranged in a comb shape at intervals are combined with a bus electrode for further collecting current from the finger electrodes. It is preferable that the current collection wiring is composed of a conductive part, a first seal part, and a second seal part. The conductive part is preferably made of a highly conductive material, and preferably made of a material containing metal. For example, it is preferable to use a material containing silver (Ag), copper (Cu), or the like. Further, the first seal part and the second seal part are provided so as to cover at least a part of the conductive part in order to chemically and mechanically protect the conductive part. The first seal portion can be made of a low melting point glass coating material. In addition, the second seal part is made of silicone rubber (oxime-removal type, low-molecular-weight siloxane reducing material, oil-resistant/solvent-resistant fluorosilicone, etc.), polyisobutylene, polypropylene, methacrylic (acrylic), polycarbonate, fluororesin (Teflon (registered trademark)). ), epoxy resin, or other resin.

Examples of the reaction substrate contained in the electrolytic solution include carbon compounds, such as carbon dioxide (CO ₂ ). Further, the electrolytic solution is preferably a phosphate buffered aqueous solution or a boric acid buffered aqueous solution. In a specific configuration example, for example, a tank containing carbon dioxide (CO ₂ ) saturated phosphate buffer is provided, and a pump pumps this solution into a channel provided between the reduction reaction electrode 10 and the oxidation reaction electrode 18. 26, and the formic acid (HCOOH), oxygen (O ₂ ), etc. produced by the reduction reaction may be collected in an external fuel tank.

The accommodating portion 28 is a member that supports the reduction reaction electrode 10 and the oxidation reaction electrode 18 and constitutes a flow path 26 through which the electrolytic solution flows. The housing portion 28 is made of a material that has the mechanical strength necessary to configure the electrochemical reaction device as a cell. For example, the housing portion 28 can be made of metal, plastic, or the like.

A separator 24 may be provided between the reduction reaction electrode 10 and the oxidation reaction electrode 18 to separate liquid and gas and allow protons to move. The separator 24 may be made of a material that can separate liquid and gas and allow protons to move, and is not particularly limited, but for example, Nafion (registered trademark), which is a solid polymer electrolyte membrane, etc. may be used. I can do it.

The reduction reaction electrode 10 and the oxidation reaction electrode 18 are electrically connected and an appropriate bias voltage is applied. The means for applying the bias voltage is not particularly limited, and examples include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, solar cells, and the like. At this time, the positive electrode is connected to the oxidation reaction electrode 18, and the negative electrode is connected to the reduction reaction electrode 10.

As shown in FIG. 2, by using a solar cell 30 as a means for applying a bias voltage, electric power is generated to be supplied to an electrochemical reaction device such as a carbon dioxide reduction device, and an electrode for a reduction reaction and an electrode for an oxidation reaction. It is possible to provide an artificial photosynthesis device including a solar battery cell. When using a solar cell as a means for applying a bias voltage, the solar cell can be placed, for example, adjacent to the oxidation reaction electrode and the reduction reaction electrode. For example, a solar cell may be placed on the back side of the reduction reaction electrode, the positive electrode of the solar cell may be connected to the oxidation reaction electrode, and the negative electrode may be connected to the reduction reaction electrode.

When synthesizing formic acid (HCOOH) or the like from carbon dioxide (CO ₂ ), water (H ₂ O) is oxidized to supply electrons and protons to carbon dioxide (CO ₂ ). At around pH 7, the oxidation potential of water (H ₂ O) is 0.82V, and the reduction potential is -0.41V (both at standard hydrogen electrode (NHE)). Also, the reduction potentials of carbon dioxide (CO ₂ ) to carbon monoxide (CO), formic acid (HCOOH), and methyl alcohol (CH ₃ OH) are -0.53V, -0.61V, and -0.38V, respectively. . Therefore, the potential difference between the oxidation potential and the reduction potential is 1.20 to 1.43V. When reducing carbon dioxide (CO ₂ ), which is a carbon compound, it is preferable that the solar cell has a structure in which 4 to 6 crystalline silicon solar cells are directly connected, or an amorphous silicon three-junction solar cell, for example. It is.

For solar cells, it is preferable to provide a window material on the light-receiving surface side. The window material is a member that protects the solar cells. The window material is a member that transmits light of a wavelength that contributes to power generation in the solar cell, and can be made of, for example, glass, plastic, or the like.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

Regarding the influence of the storage time and storage temperature after the preparation of the Ru complex polymer solution in the preparation of electrodes for reduction reactions, various electrodes in which the Ru complex polymer was supported on CP/MWCNTs sheets were prepared, and the cell characteristics were compared.

Using an ink in which 5 parts by mass of multi-walled carbon nanotubes (MWCNTs) were dispersed in a solvent (95 parts by mass of ethanol), a CP/MWCNTs sheet was prepared by dip coating on carbon paper (CP) and drying at 350 °C. did.

<Example 1>
Ru complex monomer ([Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO) ₂ Cl ₂ ]) 0.0414 g, 0.22 M iron chloride ( After preparing a Ru complex polymer solution which is a mixed solution (total volume 8.31 mL) of 1.65 mL of an ethanol solution of FeCl ₃ ), 0.33 mL of a 0.72 mM pyrrole acetonitrile solution, and 6.33 mL of acetonitrile (total volume 8.31 mL) (after the start of polymerization of pyrrole) ), the solutions were stored in the refrigerator (5°C ± 5°C) for 10 min, 2 h, 5 h, 16 h, 24 h, and 72 h, respectively, and the solutions were applied to the CP/MWCNTs sheet, dried at 30°C for 3 hours, and then Ru A CP/MWCNTs/RuCP sheet supporting a complex polymer was produced. Each of the produced CP/MWCNTs/RuCP sheets was bonded to FTO-attached glass using a carbon-based adhesive (containing conductive graphite and an acrylic polymer binder) to form an electrode for reduction reaction (active area: 1 cm). ² ).

<Comparative example 1>
The conventional manufacturing process conditions for a reduction reaction electrode for a large cell are room temperature (approximately 20° C.) and storage time of 10 minutes. Therefore, in order to investigate the influence of temperature after preparing the Ru complex polymer solution, CP/MWCNTs/RuCP sheets were prepared in the same manner as in Example 1, with storage times of 10 min, 5 h, and 72 h at room temperature (20°C ± 5°C). An electrode for reduction reaction was prepared.

Electrochemical evaluation was performed using a three-electrode cell (counter electrode: Pt, reference electrode: saturated calomel electrode (SCE)), and while bubbling carbon dioxide (CO ₂ ) gas and supplying it to the electrolyte, a potentiostat/galvanometer was used. This was done by Stat. Linear sweep voltammetry (LSV) characteristics are measured by sweeping the electrode potential from 0.6 V to -1.5 V (vs Hg/Hg ₂ SO ₄ ), and current-time (it) characteristics are measured by sweeping the voltage The measurement was performed at a constant voltage of -1.4V (vs Hg/Hg ₂ SO ₄ ) and operated for 3 hours. A 0.4M phosphate buffer aqueous solution (K ₂ HPO ₄ +KH ₂ PO ₄ ) was used as the electrolyte. After the experiment, some of the electrodes were subjected to SEM observation and X-ray diffraction (XRD) evaluation. Evaluation of X-ray diffraction (XRD) was performed using a sample horizontal strong X-ray diffractometer (manufactured by RIGAKU, RINT-TTR) under the conditions of tube voltage: 40 kV, tube current: 20 mA, and target: Cu. It was performed using the measurement method. The silicone rubber adhering to the periphery and sides of the evaluated electrode (structured with FTO glass bonded to one side of the electrode sheet for reduction reaction: evaluation surface (active surface: 1 cm ² )) was removed and an aluminum sample holder was removed. The measurement was carried out using a jig that could ensure a horizontal surface.

[result]
FIG. 3 shows the relationship between the storage time after preparation of the Ru complex polymer solution and the current value (n=2) after 3 hours of it measurement. Furthermore, FIG. 4 shows the results of the influence of storage temperature when the Ru complex polymer solution was stored in the refrigerator for 10 minutes and 5 hours, and at room temperature for 10 minutes and 5 hours.

The electrode prepared by storing the Ru complex polymer solution in a refrigerator for 5 hours showed the highest current density of 6 mA/cm ² . Compared to when stored in the refrigerator for 5 hours, the electrode produced by storing it in the refrigerator for 10 minutes to 2 hours had a 5.5 mA/cm ² (10% decrease), and the electrode produced by storing it for 16 hours to 72 hours had a 5 mA/cm ² (20% decrease). (decreased).

When the Ru complex polymer solution was stored in the refrigerator for 5 hours and at room temperature for 5 hours, the color of the solution was significantly different. When stored in the refrigerator for 5 hours, the color was orange, but when stored at room temperature for 5 hours, the color was black. Compared to conventional storage at room temperature for 10 minutes, storage in the refrigerator for 5 hours and storage at room temperature for 5 hours resulted in the effect of improving the current. However, compared to conventional storage at room temperature for 10 minutes, storage at room temperature for 72 hours resulted in 4 mA/cm ² (20% decrease), and storage in the refrigerator for 72 hours was more effective in increasing the current.

<Reference example 1>
Regarding the color of the Ru complex polymer solution becoming black when stored at room temperature for 5 hours, it was thought that the solution would change color when exposed to fluorescent lamp light at room temperature, so we separately covered the Ru complex polymer solution with aluminum foil and stored it at room temperature. Comparisons were made using samples stored for 5 hours. The results are shown in FIG. It was confirmed that when stored at room temperature, the Ru complex polymer solution turned black during the polymerization process and its properties were almost the same regardless of whether or not it was shielded from light.

[SEM observation]
Figure 6 shows the results of SEM observation of typical electrodes (stored in the refrigerator for 5 hours, stored in the refrigerator for 72 hours, and stored at room temperature for 5 hours) after the experiment (top photo: 5K times (scale bar: 10 μm), bottom photo: 30K times (1 μm)). In both cases, the Ru complex polymer aggregates on MWCNTs and is highly dispersed in aggregates of about 0.2 to 0.3 μm (white or gray in the image), and the dispersibility of the Ru complex polymer is almost the same. The surface morphology was generally the same.

[XRD evaluation]
Next, FIG. 7 shows the results of XRD evaluation of the electrodes whose electrochemical characteristics were evaluated. The upper left side of FIG. 7 shows a 20-60 degree diffraction pattern obtained by the θ-2θ method. A sharp peak was observed at 26-26.5 degrees. This peak is from the (002) plane where the carbon fibers of carbon paper (CP) are graphitized. In addition, in the XRD of CP and CP/MWCNTs that do not support Ru complex polymer, the peak intensity at 26 to 26.5 degrees is high, and the graphite (002) plane can be confirmed (upper right side of FIG. 7). The complex polymer was fine and highly dispersed, and because its amount was small, it could not be detected by X-ray diffraction.

As can be seen from the diffraction pattern around 24 to 27 degrees shown in the lower right corner of FIG. 7, which is an enlarged view of the graph on the upper left corner of FIG. 7, the peak intensity of graphite decreased after the Ru complex polymer was supported. Furthermore, due to the difference in storage time of the Ru complex polymer solution, an amorphous diffraction pattern at 24.5 to 26 degrees and a crystalline diffraction pattern at 26 to 26.7 degrees coexist. Confirmed for all electrodes regardless. The diffraction peak angle was 26.3 deg (for the electrodes stored for 5 hours and 24 hours), and the angle for the electrodes stored for other periods was lower than that, and the peak intensity was also reduced. As described above, differences in peak morphology and their order reflecting differences in the supported state of the Ru complex polymer on CP/MWCNTs were observed.

Because the peak intensity of the graphite (002) plane decreases due to Ru complex polymer support, the average compound form (crystalline carbon paper It was suggested that the difference in the state of voids (including the adhesion state of Ru complex polymer containing MWCNTs to the fibers) may be shown in the information of the diffraction pattern from the graphite (002) plane. In fact, it is thought that the carbon-based adhesive of the electrode and the FTO glass as the underlying substrate are also affected.

FIG. 8 shows the difference in the peak intensity from the (002) plane of the obtained graphite and the storage time of the Ru complex polymer solution of the electrode. Further, FIG. 9 shows values normalized based on the peak intensity of graphite (002) of 5000 CPS shown in FIG. 8 (1.0). The electrode produced by storing the Ru complex polymer solution for 5 hours had the highest peak intensity compared to other storage times. Further, as time elapsed after 5 hours of storage, the peak intensity decreased, resulting in a change trend similar to the current density of the electrochemical measurement shown in FIG. 3.

Figure 10 shows the relationship between the current value after 3 hours of IT measurement and the X-ray peak intensity of the (002) plane of graphite, and Figure 11 shows the relationship between the current value after 3 hours of IT measurement and the graphite The relationship with the normalized value based on (002) peak intensity 5000 CPS as a base (1.0) is shown. As shown in Figures 10 and 11, the current value after 3 hours of IT measurement and the X-ray peak intensity of the (002) plane of graphite or the peak intensity of graphite (002) of 5000 CPS are used as the base (1.0). There is a correlation with the normalized value. Furthermore, as shown in FIG. 12, this XRD evaluation not only examines the state of the sheet (catalyst layer) such as CP/MWCNTs/RuCP, but also the state of the sheet (catalyst layer) and the substrate (what kind of material the substrate is made of). Even if the electrode is adhesively bonded to the graphite (002), there is a correlation between the current value obtained by electrochemical measurement and the graphite (002) peak intensity, and evaluation is possible. As in the case of sheets, it can be expressed as a normalized value based on the (002) peak intensity of graphite, 5000 CPS, as a base (1.0) (see FIGS. 10 to 12).

From these results, the thicker the Ru complex polymer is deposited, the more the peak intensity of graphite is attenuated and the current density is also lowered, so the appropriate amount and supported form of the Ru complex polymer contribute to the reduction current improvement effect of the reduction reaction electrode. It is presumed that he did.

[summary]
Regarding the influence of the storage time and storage temperature after preparing the Ru complex polymer solution in the production of electrodes for reduction reactions, we prepared various electrodes supported on CP/MWCNTs sheets and compared the cell characteristics. The electrode prepared by storing it in a refrigerator (5°C ± 5°C) for 5 hours showed the highest current. Compared to storage for 5 hours, the current decreased by 10% for the electrodes stored for a short time of 10 min to 2 hours, and by 20% for the electrodes stored for 16 hours to 72 hours. Furthermore, when storing Ru complex polymer solution in the refrigerator and at room temperature (20°C ± 5°C) for 5 hours, the color of the solution differed greatly, but when used as an electrode, the effect of improving current compared to conventional storage at room temperature for 10 minutes was observed. Obtained. However, compared to conventional storage at room temperature for 10 minutes, storage at room temperature for 72 hours resulted in 4 mA/cm ² (20% decrease), and storage in the refrigerator for 72 hours was more effective in increasing the current.

From these results, the storage time after preparing the Ru complex polymer solution is required to be 5 hours or more, and assuming that the storage temperature decreases in the same way as in the case of refrigerator storage, it is thought that the storage temperature will be considerably lower in 15 hours, - It was determined that storage at 15-5°C for 5-15 hours is best.

As described above, by the method of the example, it was possible to obtain an electrode for reduction reaction that uses a Ru complex polymer and is capable of increasing the area and increasing the output.

1 Artificial photosynthesis device, 3 Carbon dioxide reduction device, 10 Electrode for reduction reaction, 12, 20 Substrate, 14 Adhesive layer, 16 Catalyst layer, 18 Electrode for oxidation reaction, 22 Oxidation catalyst layer, 24 Separator, 26 Channel, 28 Storage part, 30 solar cells.

Claims (4)

base material and
a catalyst layer containing a composite base material of carbon fiber and carbon, and a Ru complex polymer;
has
An electrode for a reduction reaction, wherein the graphite (002) X-ray peak intensity in X-ray diffraction analysis using the θ-2θ method of the catalyst layer is 100 CPS or more. base material and
a catalyst layer containing a composite base material of carbon fiber and carbon, and a Ru complex polymer;
has
The graphite (002) X-ray peak intensity in the θ-2θ X-ray diffraction analysis of the catalyst layer is different from the graphite (002) X-ray peak intensity in the θ-2θ X-ray diffraction analysis of the composite base material. , 0.02 or more. A method for manufacturing an electrode for reduction reaction according to claim 1 or 2, comprising:
A preparation step of preparing a Ru complex polymer solution containing a Ru complex monomer, a polymerization catalyst, and a solvent;
A storage step of storing the prepared Ru complex polymer solution at -15 to 5°C for 5 to 15 hours;
a coating step of applying the Ru complex polymer solution after storage to the composite base material and drying it;
A method for producing an electrode for a reduction reaction, the method comprising: The reduction reaction electrode according to claim 1 or 2,
An electrode for oxidation reaction,
A reaction device characterized by being composed of a combination of.